Recently, signals that travel on a printed wiring board, or PWB, have been transmitted with smaller amplitude in a higher frequency band.
When a PWB includes two or more layers or is otherwise multilayered, the structure of the PBW may be complex due to increases in density and number of layers of the PWB. In such PWB with a complex structure, return current paths each of which extends along corresponding signal traces may not be secured. Therefore, there is a concern that waveforms of the signals may be distorted, which can adversely affect reliability of operation of circuits or electronic components mounted on the PWB.
A related-art PWB for transmitting high-speed signals includes power patterns for power having rather stable potential and ground patterns for grounding the power, and the power patterns and the ground patterns of the related-art PWB are arranged in proximity to signal trace patterns for high frequency signals. Further, metal shielding plates may also be used to cover the PWB.
To prevent degradation of transmission quality of high-speed digital signals, a connector-conversion adapter for CD-ROM device employs a PWB having the above-described construction so that a positioning hole of the PWB may be formed according to a through-hole structure and a metal film that forms the positioning hole may be connected to multiple ground patterns. Specifically, the metal film forming a part of the through-hole structure of the CD-ROM device is connected to three ground patterns, which have been separately connected, so that the metal film of the through-hole structure may be included in the ground patterns. By so doing, a ground potential is enhanced to prevent high-speed digital signals from being degraded while being transmitted through the connector-conversion adapter.
It is required that signals traveling over the PWB further increase their transmission speed. To meet such demand, recent PWBs intend to employ a transmission method for high-speed differential signals, such as Serial-ATA and PCI-Express. As noted above, for multilayer PWBS, the structure thereof has become more complex due to increases in the density and number of layers, and therefore it has been difficult to secure return current paths extending along respective signal traces, which can adversely affect the operating reliability of the circuits or electronic components mounted on the PWB.
To eliminate the above-described drawbacks, multilayer PWBs for carrying high-speed signals include a power layer that holds rather stable potential and a ground layer that is arranged in proximity to a layer having signal traces for high frequency signals so as to secure the return current paths. When the PWB includes a double-sided structure, the ground pattern may be provided on a back side of the PWB. When the PWB is multilayered, the ground pattern may be provided on a second, intermediate layer, which is an internal conductive layer.
With the above-described structure of the PWB, when the length of a lead pattern is long, waveforms may degrade during signal transmission. For example, when signals that are transmitted at speeds of Gbps travel over an inappropriate layout of ground of a PWB, wiring delay and potential difference may be caused due to inductance in a lead wire between a ground pad of an electronic component and a via connecting to an inner layer ground pattern. Consequently, high speed differential signal traces on the PWB may experience increased loss of fidelity to waveforms in signal transmission in the high-frequency band, and therefore the output waveforms may be degraded and not have fidelity in signal transmission.
Further, regarding a distance from a lead or via to a trace pattern, when an extended end portion (including the lead wire or via) of a return current path with respect to a signal trace for high-speed differential signals is laid out carelessly, the equilibrium of high-speed differential signals may be lost, and therefore the quality or fidelity of the waveform may deteriorate.
With the construction previously described, the ground can be made stronger but the inductance from the ground pad to the through hole cannot be removed.